The invention relates to a gas filter element.
Such filters are used for the separation of floating particles from an air stream such as the air entering ventilating or air conditioning equipment or being recirculated thereby.
For the purpose of separating dust from an air stream it is known to use filter units containing as the filtering medium nonwoven mats of glass or textile fibers. It is advantageous for such nonwoven mats to be used in stiff supporting lattice designs known as high surface area filter units, in which the edges of the pieces of mat inserted in a V-shaped configuration are clamped to the holder in a dust-tight manner by appropriately shaped grids. A filter of this kind is described, for example, in German Petty Pat. No. 6,908,374. Due to the stiff mounting of the filter media, such high surface area filter designs have particularly good characteristics with regard to degree of dust separation, dust holding capacity and dust adhesion. Their use, however, is uneconomical inasmuch as the mountings are very expensive. Furthermore, the relatively great amount of time required for the replacement of the dust-filled pieces of matting with clean ones constitutes a disadvantage. Furthermore, in all such filter elements considerable difficulty is involved in making sure that the installed mat sections are joined together in a dust-free manner along the edges, for otherwise dust leaks can occur.
In recent times filter elements have become known which have an externally similar configuration while dispensing with stiff supporting structures.
For the manufacture of such a filter element from cut-to-shape pieces of glass fiber or textile fiber matting, such pieces have heretofore been assembled by sewing, cementing or spot welding to form the actual filter pockets. Various numbers of these filter pockets are joined removably or irremovably to a front mounting frame. The element is commonly used as a ready-assembled unit. Such filter elements have not, however, been widely used. The individual filter pockets balloon under operating conditions, resulting in irregular distribution of the flow on the active filter surfaces. Fluttering occurs, and this results not only in an unsatisfactory separation of dust particles but also the danger of damage to the filter pockets as well as the danger of contamination of the filtered air by the filter fibers. It has furthermore been found that the known outer stitching of the seams gathers the edges of the mats imperfectly, so that edge piping must be provided to protect them. In addition--especially in the case of fine and ultra-fine filter elements--the unavoidable holes created by the penetration of the needles have to be cemented shut by expensive hand operations.
To prevent the filter pockets of an element from touching one another when they balloon under operating conditions, thereby impairing their efficiency, a variety of methods has been used to restrain the ballooning action, but none of them has been entirely satisfactory. When the opposite sides of the filter pocket are joined directly to one another by tack stitching or continuous stitching, the areas where the sides are joined to one another are compressed together, and this reduces the effective filter area, even though each pocket is prevented from ballooning against the adjacent pocket. Furthermore, the flow of air into the pockets is reduced and the resistance of the element to the passage of air is increased.
Where the method of joining together the opposite sides of the filter pocket leaves space between the opposite seams, better conditions are achieved with regard to air flow, but there are other disadvantages: if the sides of the filter pocket are tied together spot-wise, any fluttering or vibrational movements between them will threaten to tear them away from each other at the points where they are attached. To prevent this, gores have been sewn between the sides of the filter pocket, i.e., triangular pieces of fabric are sewn between the sides of the filter pocket from the mouth to the bottom thereof, with the apex of the triangle at the bottom of the pocket. The disadvantage of this method of stitching is the great amount of time required for the sewing operation and for the subsequent gluing or sealing shut of the perforations made by the needle. Also, at the apex or bottom end of the pocket, there is a decided reduction of the active filter surface.